So, both Kira and the "crystal" engulfing her are actually the Founder. Which means that when she shot the crystal with her phaser, she was shooting herself. Yet in "Crossover", we see that a single shot from a Bajoran phaser is enough to blow Odo into little itty bitty bits. And later on we will learn that even a low level phaser shot is painful enough that it makes a Founder lose whatever shape they are in. So how can the Founder shoot herself?

Plotline

When Kira and Odo chase a Maquis operative to a desolate planet, they quickly lose him in a network of caves. When Kiras foot becomes trapped in a crystal of some sort, they assume that it will be simple to free her - but all their efforts fail, and the crystal gradually engulfs Kira. Is this the end for the love of Odo's life?